1. Field
An aspect of the present disclosure relates to a resonator and an optical sensor using the same, and more particularly, to a semi-circular resonator using a whispering gallery mode (WGM) and an optical sensor using the same.
2. Description of the Related Art
A whispering gallery mode (WGM) has the form of a wave that is continuously totally reflected from an inner surface of a spherical or circular resonator to circulate in the resonator. In general, the intensity of a wave in a free space decreases in proportion to the square of a distance from an epicenter. However, the intensity of the WGM decreases in proportion to a distance in a thin layer adjacent to the inner surface of the resonator, which has a characteristic similar to that when a wave advances along a linear waveguide.
As a result, the resonator using the WGM has a high quality factor (Q=λ/Δλ), a low mode volume, and a small size. Hence, the resonator using the WGM is used in various sensors and has aroused a lot of interest.
Nevertheless, a majority of optical sensors using typical resonators are passive sensors and require external light sources. Therefore, optical coupling with fine alignment between an input/output optical fiber and a resonator is required to achieve an optical connection between an external light source and the resonator.
A typical resonator is based on a circular shape, and hence, when an optical fiber is optically coupled to the resonator, the optical fiber should be finely adjusted to μm or less in terms of the resonator. In addition, if the position between the optical fiber and the resonator is changed, the Q-factor of the resonator is changed. As a result, an optical sensor using the typical resonator cannot perform measurement even when a minute vibration occurs. Hence, the optical sensor may be used in a laboratory, but there is a limitation in commercialization.
In the case of a PLC-based ring or disk resonator, an optical coupling waveguide for evanescent waves of the resonator can be integrated, and stable characteristics can be ensured. Thus, the resonator has been partially commercialized. However, the Q-factor of the resonator is lowered due to optical coupling, and there is a limitation in ensuring uniformity.
In the case of a deformed micro-disk resonator in which oscillation in a single direction is implemented by deforming the shape of an extremely small portion of a circular resonator, the Q-factor of the resonator is not lowered due to optical coupling between an input/output optical fiber and the resonator, and fine alignment is not required. Thus, it is possible to implement a resonator having a high Q-factor. However, there is required a technique for designing a very fine resonator, and the tolerance in the manufacturing of a designed resonator is very small, which requires a high-level lithography and etching technique. In addition, as there occurs very low outputs that are not adjusted due to characteristics of the resonator having a high Q-factor, signal processing is problematic when the resonator is practically applied.